A rubber sponge having porous structure is suitably used for, for example, various rollers for an image forming apparatus, a sponge for washing, a sponge for absorbing fluids such as water, oil and the like, a material for stamp, and the like.
Such rubber sponges can be produced, for example, by a foam molding method using a blowing agent, a method using hollow microparticles or the like, an elution method to elute soluble particles mixed in advance, or the like. For example, as a “method for forming a conductive compressible layer 23” of an image transfer roll for an image forming apparatus, Patent Literature 1 discloses “A foam molding method in which a blowing agent is compounded in a synthetic rubber formulation to form a compressible layer, and the blowing agent foams during rubber vulcanization to form a compressible layer having cells; a hollow microparticle mixing method in which hollow microparticles are compounded in place of a blowing agent, and independent cells are formed; or a powder elution method in which powders soluble in an elution liquid such as water, methanol, etc., for example, sodium chloride, sugar and the like, are compounded in a synthetic rubber formulation, and the powders are eluted after rubber vulcanization to form a compressible layer having cells; and the like are known.”
More specifically, as a method using microballoons, Patent Literature 2 discloses “A pressure roller having an elastic layer dispersedly containing cavity portions formed by resin microballoons around a core metal, wherein the elastic layer is formed by heating a liquid silicone rubber containing expanded resin microballoons on the core metal at a temperature lower than the softening point of the resin microballoons to cure the liquid silicone rubber, and then breaking the resin microballoons to form the cavity portions” (claim 1, etc.).
In addition, Patent Literature 3 discloses “A substantially closed cell type silicone elastomer porous body, which can be used for image forming components such as a copying machine, a laser printer, or the like by using a water-in-oil type emulsion composition capable of producing a silicone elastomer porous body having uniform fine cells (bubbles) without a foaming phenomenon”.
In addition to various types of rollers for image forming apparatuses, Patent Literature 4 relating to “sponge rubber print body” as a material for stamp, discloses “A sponge rubber print body having open cells obtained by kneading rubber, water soluble fine powders, a vulcanizing agent, a filler and a staple of organic synthetic fibers of a fiber length of 0.2 to 2 mm to prepare a master batch, and then vulcanizing the mixture and washing out the water soluble fine powders” (Claim 1).
Patent Literature 5 discloses a silicone rubber sponge produced by mixing expanded resin microballoons with a low molecular weight silicone rubber to prepare a compound A, mixing a high molecular weight silicone rubber with the compound A to prepare a silicone rubber compound, and heating the silicone rubber compound at a temperature lower than a softening point of the resin microballoons to cure the silicone rubber (see claim 1).